1. Field of the Invention
The present invention relates to an image forming apparatus, particularly to an image forming apparatus that develops an electrostatic latent image with toner.
2. Description of Related Art
In a conventional image forming apparatus, a developing roller supports a developer composed of magnetic carriers and non-magnetic toner, and supplies the non-magnetic toner to a photosensitive drum with an electrostatic latent image formed thereon so as to develop the electrostatic latent image. In the image forming apparatus, a developing bias is applied to the developing roller so as to form an electric field for movement of the toner from the developing roller to the photosensitive drum.
With respect to the developing bias application, there are two methods, namely, a DC application method and an AC application method. In the DC application method, a DC voltage is applied as the developing bias, and in the AC application method, a superimposed voltage of a DC voltage and an AC voltage is applied as the developing bias. The AC application method permits more faithful development of the electrostatic latent image on the photosensitive drum than the DC application method. Accordingly, an even and smooth toner image (a toner image with favorable graininess) can be formed by the AC application method.
As a conventional image forming apparatus, for example, an image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2002-258588 is known. In the image forming apparatus, the load current for an AC component in a developing bias applying means is detected, and at least one of the frequency and the voltage of a square-wave AC voltage outputted from an AC voltage generating means is changed in accordance with the detected load current. Thereby, the pulse wave of the developing bias voltage applied to a developing device is prevented from overshooting and undershooting. In the image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2002-258588, however, carrier adhesion to a toner image occurs with a change in a DC developing bias voltage.